WASHINGTON (AP) — The State Department released fewer Hillary Rodham Clinton emails than expected Friday, saying its process was slowed by intense scrutiny from U.S. intelligence agencies to ensure that emails from her private server don't contain sensitive or classified government secrets.

In emails made public Friday, the government censored passages to protect national security secrets at least 64 times in 37 emails. Clinton has said she never sent classified information from her private email server, which The Associated Press was first to identify as operating in her home in New York.

The government's release Friday of 2,206 pages of emails brings the number of emails publicly released by the State Department to roughly 12 percent of the 55,000 pages Clinton had turned over to department lawyers earlier this year. That falls short of the 15 percent goal set by a court ruling in May, a lag the State Department attributed to interest by the inspector general of the U.S. intelligence community into the possible compromise of classified information.

Memos sent by the inspector general, I. Charles McCullough III, alerted the FBI to a potential security violation arising from Clinton's use of a private server located in her home, a counterintelligence referral the office says it is required to make under law.

The inspector general said his office has found four emails containing classified information while reviewing a limited sample of 40 of the emails provided by Clinton. Those four messages were not marked as classified but should have been handled as such because they contained classified information at the time they were sent, the inspector general said.

Those emails "should never have been transmitted via an unclassified personal system," according to a statement the office released jointly with the State Department inspector general, Steve A. Linick, last week.

Clinton has repeatedly defended her email usage, saying her private server had "numerous safeguards" and placing responsibility for releasing the documents on the State Department.

"They're the ones that are bearing the responsibility to sort through these thousands and thousands of emails and determine at what pace they can be released," she said after meeting with labor leaders Thursday in Maryland. "I really hope that it will be as quickly as possible."

At least some members of Congress were communicating with Hillary Rodham Clinton using her private, non-government email address.

One message released Friday showed an exchange with Sen. Barbara Mikulski, D-Md., in May 2009 under the subject line "Aid dir." The substance of the messages was censored for reasons of personal privacy and to protect confidential deliberations, but it was sent the same day a senior director for Guyana at the U.S. Agency for International Development was sworn in. Clinton wrote to the senator, "Barb, I will call to explain."

Clinton's decision not to use a State Department email account has become a political problem for her, as Republicans seize on the disclosures to paint her as untrustworthy and willing to break rules for personal gain.

Sens. Charles Grassley, R-Iowa, chairman of the Senate Judiciary Committee, and Ron Johnson, R-Wis., chairman of the Senate Homeland Security and Government Affairs Committee, said they were concerned that Clinton's attorney, David Kendall, apparently holds thousands of Clinton's emails — including some that may contain classified information — on a thumb drive at his Washington office.

"It's a serious breach of national security if the United States government fails to secure classified material in the hands of people not authorized to possess it, no matter who they are," Grassley said in a statement. "There are fundamental questions as to what the FBI is doing to securing these classified emails and why the State Department is not fully cooperating with the inspectors general at the State Department and the intelligence community to ensure that all of the appropriate emails are identified."

Grassley wrote a letter to FBI Director James Comey asking him to explain what the FBI is doing to ensure that classified information contained on Kendall's thumb drive is secured and not further disseminated.

Johnson, meanwhile, wrote a letter to Kendall asking what steps he's taken to ensure classified information is not released.

"The lax storage and safeguarding of this information could have serious consequences to national security," Johnson wrote Wednesday in a letter to Kendal. The letter asked Kendal to answer eight specific questions, including how he secured the thumb drive and naming all individuals who had access to it.

Previous emails released by the agency revealed that Clinton received information on her private account about the deadly attack on U.S. diplomatic facilities in Benghazi that was retroactively classified as "secret" at the request of the FBI. Her correspondence also contained several messages that were deemed sensitive but unclassified, detailed her daily schedule and contained information — censored in the documents as released — about the CIA that the government is barred from publicly disclosing.

There was no indication from emails released so far that Clinton's home computer system used encryption software to communicate securely with government email services. That would have protected her communications from the prying eyes of foreign spies, hackers, or anyone interested on the Internet.

Earlier this year, a district court judge mandated that the agency release batches of Clinton's private correspondence from her time as secretary of state every 30 days starting June 30.

The regular releases of Clinton's correspondence all but guarantee a slow drip of revelations from the emails throughout the Democratic presidential primary campaign, complicating her efforts to put the issue to rest. The goal is for the department to publicly unveil all 55,000 pages of her emails by Jan. 29, 2016 — just three days before Iowa caucusgoers cast the first votes in the Democratic primary contest.

A Republican-led House panel investigating the deadly 2012 attacks in Benghazi, Libya, also is examining emails of Clinton and other former department officials, raising the possibility of further revelations into 2016.

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Associated Press writers Eric Tucker in Washington contributed to this report.

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